The present invention relates to a zoom lens barrel, and more particularly to a zoom lens barrel having incorporated therein an optical system including a first lens group which is movable along the optical axis for both focusing and zooming by a single operating ring.
A conventional zoom lens barrel of this type is disclosed in Published Examined Japanese Patent Application Sho 51-4852. The lens barrel houses an optical system in which a first lens group alone is used for focusing, while second and third lens groups are axially moved only for zooming. Because of this construction of the optical system, the lens barrel has the drawback of being exceedingly large in overall length and very inconvenient to carry around and handle for taking photographs.
Published Unexamined Japanese Utility Model Application Sho 53-82341 discloses another zoom lens barrel in which a first lens group is movable directly by the axial movement of a single operating ring and a second lens group is movable by a cam ring turned by the movement of the first lens group to effect zooming.
FIG. 1 shows such a conventional construction including a first lens group 1 and a second lens group 2. The first lens group 1 is moved for focusing, while the first and second lens groups 1 and 2 are moved for zooming.
A stationary cylinder 3 rotatably supports a cam ring 4. Indicated at 5 is a first lens holding frame, and at 6 a second lens holding frame. The first lens holding frame 5 is fitted in a first lens moving frame 7 by helicoidal screw engagement as at 8 and is integral with a single operating ring 9 around the stationary cylinder 3. The first lens moving frame 7 and the second lens holding frame 6 are fitted in and supported by the cylinder 3 to be rotatable and axially movable. Indicated at 10 is a scale ring.
When the operating ring 9 is turned, the first lens group 1 is moved along the optical axis for focusng by virtue of the helicoidal engagement. At this time, the scale ring 10 is also turned with the operating ring 9 by the engagement of a pin 12 in an axial groove 11, but since a guide pin 14 on the first lens moving frame 7 is engaged in a circumferential groove 13 in the inner periphery of the scale ring 10, the axial movement of the first lens group 1 (the turn of the operating ring 9) does not move the scale ring 10 axially.
On the other hand, the operating ring 9, when moved axially, axially moves the first lens moving frame 7 by means of the helicoid 8. The moving frame 7 has a guide pin 17 engaged in a cam groove 16 of the cam ring 4 through an axial groove 15 in the stationary cylinder 3, while the second lens holding frame 6 has a guide pin 20 engaged in another cam groove 19 in the cam ring 4 through another axial groove 18 in the cylinder 3. The first lens moving frame 7 has an axial groove 21 receiving therein a key-shaped projection 22 of the second lens holding frame 6. Consequently the axial movement of the first lens moving frame 7 turns the cam ring 4 and axially moves the second lens holding frame 6 alike. With the axial movement of the first lens group 1, therefore, the second lens group 2 is axially moved for zooming by being governed by the cam grooves 16 and 19.
For illustrative purposes, FIG. 2(a) represents the zooming movement of an optical system which is housed in a lens barrel with a single operating ring and in which a first lens group is movable for both focusing and zooming. In the graph, the focal length F (ranging from short length S to long length L) is plotted as ordinate vs. the amount of axial movement, A, of each lens group as abscissa. Now assuming that the first lens group 1 is moved from the short focal length position S to the long focal length position L at a speed represented by a line B, the second lens group 2 moves from S to L in a direction opposite to the movement of the first lens group 1 at a speed indicated by a curve C. FIG. 2(b) shows the ratio R of the speed of the second lens group 2 relative to the speed of the first lens group 1. With an optical system involving a high zooming ratio, the speed ratio is as large as about -10 especially toward the long focal length position. Thus to give a large zooming ratio, the amount of rearward movement of the second lens group must be greatly increased relative to the amount of forward movement of the first lens group 1.
With lens barrels as shown in FIG. 1, however, the speed ratio is usually limited to about -1 in view of operability although the limit ratio may somewhat vary with the weight of the movable lenses, finish of the parts and quality requirements. In the case of the optical system represented in FIGS. 2(a) and (b), therefore, extreme difficulties are encountered with the known techniques in manufacturing lenses which will afford a high zooming ratio. Thus to obtain an appropriate speed ratio (e.g. of up to -1), there are great limitations on the optical design, with the necessity of decreasing the zooming ratio and reducing the refractory power of the first lens group. The optical system will then become larger in its entirety. The increase in the overall size of the optical system will lead to an increase in the manufacturing cost of the lenses and barrel, result in disadvantages to the operating characteristics of the mechanism and render the product heavier, larger and difficult to handle.